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### Pressure- Introduction.

Blood pressure is measured with a sphygmometer while the tyre pressure guage uses the Bourdon guage method. Digital meters are also available now.

Blood pressure is given in mili meters of mercury (Hg), while the air pressure of a tyre is given in p.s.i. pounds per square inch, which is no longer used in Physics.

Let us start this chapter with an activity. You can do this alone or with a friend without the use of any apparatus or material. What you have to do is shown in the Figure.

Fig.1. Learning the difference between Force and Pressure.

First you can press your arm, or somebody else’s arm with your palm as shown in 1.a.  Just have an idea how you felt it. Then you can press the arm at the same place, with the same force using only a finger as shown in Fig 1.b.

Ali and Nelly did this a few times. They came to the conclusion that the finger causes more pain than the palm.

What could be the reason?

The above activity clearly shows that the force and pressure are not the same.

What is pressure?

Pressure is how a force acts on a certain area. The cross section of a finger is very small compared to the palm. When pressing with the finger the area in contact is about 1square centimetre. (1 cm2). A child’s palm may have an area of about 40 cm2.

Assuming that the force given is 30 N, can you determine the pressure at 1. a. and 1.b.?

Calculation.

 Pressing with the palm. Pressing with the finger. Pressure = Force/ Area. Pressure =  30 N / 1 cm2 Answer  = 30Ncm-2 Pressure = Force/ Area. Pressure =  30 N / 40 cm2 Answer  - 0.75 N cm -2

Pressure in solids Fig.2 The cat and the block.

The Fig.2 shows a cat playing with 3 identical solid blocks. Each square drawn on the box is 1 cm2.

Each block has a mass of 24g. This may be considered as 24 gram weights. As Force (weight) is mg. this is equal to 0.2 N. Find the pressure under each block.

Worked out example.

We shall calculate the pressure under ‘A’.

Calculating pressure under ‘A’.

Weight of block  .   .    .  = 0.2 N.

Area under the block       =  2 cm2.

Pressure under the block = 0.2/2   = 0.1 N.cm-2.

If you are clever you will work out the pressure under ‘B’ and “C’.

Liquid Pressure

In a liquid, pressure can be measured at any depth. Pressure increases directly with the height or depth. Irrespective of the volume of the liquid or the area at the base, at a particular depth the pressure remains the same. But the density of the liquid affects the pressure.

The measuring cylinders given contain three different liquids. Their densities are given. Let us calculate the liquid pressure at the bottom.

Fig,2.Calculating the pressure at any depth.

Calculating the liquid pressure  for oil.

1.   The height of oil    .     .      .     = 3.0 cm.

Density of oil   .       .        .       = 0.7 g.cm-3

Gravitational attraction.     .      = 0.0098 N per gram.   (1 Kg weight = 9.8 N)

Formula for liquid pressure.     = height x density x g.

= 3 x0.7 x 0.01, N cm-2 =  0.02 N cm-2

Calculating the liquid pressure for water.

2.  Formula for liquid pressure = h d g.

= 4x 1x0.01   =   0.04 cm-2

In the same manner you should be able to calculate the pressure for the Juice,

Gas Pressure.

This is usually measured by the height of a mercury column or a water column that can balance the gas pressure.

Atmosphere consists of a gaseous mixture. So it should exert a pressure on us from all sides. This can be conveniently measured by using a liquid of high density. The density of mercury is 13.6 times that of water; so it is the most suitable for the purpose.

A thin glass tube is completely filled with mercury and inverted in an open cup of the same liquid. There should be a good knowledge of the technique to fill the tube so that there wont be any air bubbles inside. Then the tube is inverted taking care not to allow any air enter. The height of the column will change according to the out side pressure.

Normal atmospheric pressure at sea level is 76 cm of mercury.

Try converting this to Pascals. (Pa).

A pressure of 1N on an area of 1m2 is = 1  Pa.

This is called a Barometer.